This disclosure is related to projection television systems, and is particularly concerned with improving projection screens for such systems.
In many projection television systems, the image is projected onto the rear of the screen, and is viewed from the front of the screen. Embodied in the screen elements, which normally comprise one or more transparent sheets, usually of plastic, are lens elements in the form of Fresnels and lenticles, for example, which provide for focusing the image toward the viewer, and for enhancing its brightness. Ambient light reflected from the viewed surface of the screen reduces the apparent contrast of the image. To reduce the reflections to a tolerable minimum, light-absorbing elements are commonly applied to the surface of the screen on the viewer side. Such elements are typically applied to discrete areas of the screen where there is no image transmission. The elements may be in the form of black stripes running horizontally or vertically, or of black elements of other configurations corresponding to non-image-transmitting areas of the front screen surface.
The light-absorbing elements must provide for positive, permanent adherence to the surface of the screen, and preferably have a surface providing for minimum reflection of ambient light. Further, the method of application must lend itself readily to the screen manufacturing process, and be easy and simple to apply on a mass-production basis.
Viewing screens comprising a transparent substrate having a multiplicity of light-focusing optical elements on one surface of the sheet, and a planar surface on the opposite, or viewing, side, are well known in the art. An example is disclosed in U.S. Pat. No. 1,924,841 issued to Shimizu in 1934. The dark elements are typically applied in registration with the light image-transmitting areas through the screen by optical elements between the screen and the image source. This technique is described in Shimuzu and others.
In U.S. Pat. No. 3,832,032, Shimada discloses a rear-projection screen in a projector which uses a cathode ray tube as an image source. The screen has on one side a multiplicity of parallel lenticular ridges, while the other side of the screen has a plane surface provided with a plurality of light diffusing, stripe-like areas arranged alternately thereacross, and made by sandblasting. The lenticular ridges and the light diffusing stripe-like areas cooperate to provide light emerging from the screen with a horizontal diffusing angle that is said to be controllably greater than the vertical diffusing angle thereof. The light-absorbing stripes are said to be operative to absorb ambient light and thereby avoid deterioration of the contrast of an image projected through the screen onto the plane surface thereof. A black, light-absorbing material is coated over the planar surface of the sheet except in those areas where the light focused by the lenticular ridges falls on the front surface of the screen. The coating is accomplished by photographic means wherein a photosensitive material deposited on the front, planar surface of the screen, is exposed to light from the rear of the screen which passes through the lenticular, focusing ridges. The photosensitive material is developed, leaving bare areas on the front surface of the screen for receiving the deposit of light-absorbing material, which is, for example, a "black pigment."
Nishimura et al discloses in U.S. Pat. No. 4,309,073 a screen assembly comprising a first layer of translucent resin having a first surface formed with a Fresnel lens, and a second surface. A second layer of translucent resin is mixed with a diffusion element such as a pigment. The second layer is integrally provided on the second surface of the first layer. The translucent resin is an acrylic. The diffuser is a thin layer of isotropic material formed by a pigment mainly consisting of SiO.sub.2.
In U.S. Pat. No. 3,598,640, Bennett discloses a rear-projection screen which includes on the image-receiving face thereof a coating comprising a transparent organic vehicle and a small portion of a light-diffusing silica material. The silica material is preferably a synthetic, amorphous silica having generally micron-sized particles. Substantially all of the particles include capillary passages therein. A substantial proportion of the silica particles are held in place by a transparent organic vehicle in a manner such that at least portions of the particles are exposed to atmosphere, rather than being completely embedded in the vehicle. The benefit is said to be that a much greater light reflectivity and diffusion occurs at the air-particle interface than at a coating-particle or vehicle-particle interface, and that the capillary passages in the silica particles present a large interfacial area.